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The smallest quantity of liquid in the metric system is one milliliter, usually written as 1 ml, which is a thousandth of a liter. A liter is the base measure of liquid and is the equivalent of 32 fluid ounces, one quart or about four cups. One cup of liquid is about 250 ml. Dry quantities vary from liquid quantities in the metric system and are measured in grams and kilograms. A typical cup of flour is equivalent to nearly 250 grams, and a kilogram of all-purpose flour is equivalent to about eight cups.

A teaspoon holds 5 milliliters. A tablespoon, which is 3 teaspoons, holds 15 milliliters. Converting from one form of measurement to another in cooking is becoming a more common practice with more people sharing recipes on the Internet.

The number of milligrams in 1 teaspoon varies according to the density of the substance being measured. Milligrams are a unit of weight; the teaspoon is a unit of volume equivalent to 5 milliliters. These measurements are part of the modern metric system established by the International System of Units.

A gram is a measurement of mass, or the amount of matter something contains, and a teaspoon is a measurement of volume, or the amount of space something takes up. Therefore, it isn't possible to precisely convert teaspoons into grams unless the density of the substance being measured is known.